This invention is directed to concrete products incorporating a lightweight aggregate consisting primarily of bottom or base ash recovered from the combustion of pulverized coal in a dry bottom type furnace and to such bottom ash and to a method of producing the same for use as a lightweight aggregate in concrete products.
A wide variety of materials has been used in the past as lightweight aggregates in the manufacture of concrete products. Such aggregates as are used in ready-mix concrete must meet ASTM:C330 specifications. When lightweight aggregates are used in hollow-load bearing concrete masonry units, commonly known as concrete blocks, they must meet ASTM:C331 specifications. When a lightweight aggregate is used in insulating concrete it must meet ASTM:C332 specifications.
Natural occurring materials which have been used as lightweight aggregates include pumice, scoria, and volcanic ash. Also, various processes have been devised for expanding, calcining, or sintering such products as furnace slag, clay, diatomite, perlite, shale, slate, vermiculite and fly ash. All of these products have been used in varying extents as lightweight aggregate materials, and their use has depended to a large extent on their relative availability as well as the cost of handling, preparation, and delivery. The expanding, calcining, or sintering procedures require the availability of energy in the form of heat or steam, and particularly the calcining and sintering operations require substantial quantities of heat at relatively high temperatures to effect the modifications or change in the structure to make the product useful for masonry or concrete products, to eliminate deleterious substances such as organic impurities, iron oxide, and unburned carbon. Examples in the patent literature of such substances include:
U.S. Pat. No. 2,105,324: cinders, puffed slate, bloated or burned clay or shale, popcorn-slag, dry granulated slag.
U.S. Pat. No. 1,800,024: volcanic ash, ground cinders, fine ground pumice or sand.
U.S. Pat. No. 3,096,188: granular slag.
U.S. Pat. No. 3,362,837: expanded pumicite.
U.S. Pat. No. 3,245,812: ground oil shale ash.
Certain by-products of the burning of coal have been used widely as lightweight aggregate materials. One of the earliest coal products used was the clinker or cinder formed from burning lump coal on grates in broiler furnaces or the like, as disclosed in U.S. Pat. Nos. 89,311 of 1869, 145,277 of 1873, 642,301 of 1900, 664,710 of 1900, 770,083 of 1904, and 991,419 of 1911. More recently the available cinder product has been a by-product of burning crushed coal on travelling grates, under-fired grates or throwing stoker systems. Cinders, as a concrete aggregate, have fallen generally into disuse since the concrete products, particularly concrete blocks made with cinders, are unsuitable for outside use due to their porosity, low compressive strength, and their tendency to stain. ASTM:C90 defines the specifications for concrete block, and cinder blocks have been generally relegated to grade S, that is, limited use above grade in exterior walls with weather protective coating and in walls not exposed to the weather.
In the 1920's a more effective method of firing power plant boilers came into use consisting of the pulverizing of the coal into a fine powder, the addition of a primary air, and the burning of the coal powder substantially in a suspended state within the furnace. Such coal is pulverized in a rolling mill to the point where approximately 70 to 80 percent passes through a 200-mesh screen, is conveyed from the pulverizer with air into the furnace, and combustion takes place almost instantly while the fine coal particles are in a suspended state. This method of burning coal has come into wide use throughout the country due to it increased efficiency of combustion. The ash content of the coal, which may vary from a low of about 4 percent to a high of about 20 percent or more, is subject to the intense heat of combustion which may run between 2000.degree. and 2800.degree.F. Most of the ash is in the form of fly ash: the discrete sphere-like particles which are convected upwardly with the flue gases and are separated therefrom by electrostatic and/or mechanical collectors. The production of the large quantity of fly ash has presented substantial disposal problems to the public utilities since only a small part of this material is commercially used. The remaining or unutilized fly ash is frequently carried by water into a large holding or sluicing pond where the material settles and may later be removed by dump trucks to a land fill. Due to the nature of the fly ash particles, it has found use both as a pozzolan and as a lightweight aggregate. In the latter form it has been pelletized, sintered, and then crushed and screened to provide proper gradation and is a good aggregate material.
However, the manufacture of a lightweight aggregate from fly ash requires the employment of energy as well as machinery for pelletizing, or extruding, drying, and then firing on a sintering grate at approximately 2200.degree.F. The resultant product is similar in nature to expanded shale and clay. U.S. patents which deal with the use of fly ash alone or in combination in concrete products and with the treatment of fly ash for its use in concrete products include the following:
U.S. Pat. No. 2,250,107 of 1941: fly ash used in its natural state in large quantities as a substitute for cement. Examples are also given where cinders were added as coarse aggregates.
U.S. Pat. No. 2,987,408 of 1961: forming a pozzolanic material using a specific magnetic separation to remove magnetic particles.
U.S. Pat. No. 3,030,222 of 1962: fly ash nodulized with a binder of sewage sludge and fired.
U.S. Pat. No. 3,192,060 of 1965: combining fly ash with alkaline earth reactants, pelletizing, and curing to form a lightweight aggregate.
U.S. Pat. No. 3,328,180 of 1967: pelletizing fly ash, drying the pellets, firing the pellets to an ignition temperature, cooling, crushing and refiring at a temperature between 1000.degree. and 2000.degree.F.
U.S. Pat. No. 3,669,701 of 1972: an oil well cement uses hollow cenospheres (float ash) as a lightweight aggregate.
U.S. Pat. No. 3,759,703 of 1973: mixing fly ash with calcium carbonate, firing and heating to a temperature of at least 1500.degree.C to form a molten slag, water quenching, and grinding to obtain a hydraulic binder.
U.S. Pat. No. 3,782,985 of 1974: using fly ash cenospheres as a lightweight aggregate.
Boux, "Canadians Pioneer New Fly Ash Processing System", Minerals Processing, March 1969, (pp. 16-19).
When coal is burned in pulverized form in a furnace, such as that used in the generation of steam at power plants, a major portion of the ash content of the coal is expelled as fly ash while a minor portion of the ash content, which may run as high as 40 percent, for some types of coal or furnaces, down to as low as 10 percent with other coals or furnaces, collects on the furnace walls or falls or runs to the furnace bottom and it is collected and removed from the furnaces by various arrangements. Two types of pulverized coal burning furnaces have come into general use and include the slag or wet bottom furnace on the one hand and the dry bottom furnace on the other hand. Each produces a distinctively different bottom ash.
In the slag or wet bottom cyclone furnace, the ash content which is collected at the bottom of the furnace is maintained in a liquid condition by maintaining a temperature on the slag well above its fusion temperature. Such slag temperatures may be from 1900.degree. - 3000.degree.F. A slag tank or water filled pit is positioned below the furnace to receive the tappings of molten slag from the furnace.
The resulting product has become known in the industry as wet bottom boiler slag or bottom slag. It is glassy and angular in appearance, resembling angular crushed dark-colored glass. Boiler slag has found extensive use in asphalt paving mixtures and has been promoted as an aggregate for surface de-slicking in bituminous concrete. It has also been used as a convention aggregate in the manufacture of concrete blocks and as a sandblasting material. It is usually relatively heavy in weight, exceeding 65-70 lbs. per cubic ft., and for this and other reasons has not been suitable as a qualifying lightweight aggregate as defined by relevant ASTM specifications.
The dry bottom furnace, formed with a hopper bottom, has a sufficient cooling surface so that the ash which impinges on the furnace walls or on the hopper bottom is solid and is essentially dry. These furnaces generally have an open grate at their bases and below the open grate there is generally provided a water-filled ash pit to receive the ashes from the furnace.
A certain amount of molten slag will also form on the internal walls of the boiler and will find its way into the ash pit. However, a large portion of the dry bottom ash is collected in an essentially dry state and thus has physical characteristics which distinguish it from wet bottom slag. This is due to the fact that during the combustion of pulverized coal, some of the molten ash particles are heavier and fall to the bottom where they cool and are collected dry in the form of discrete spheres. Other smaller molten ash particles tend to agglomerate and fuse together and thus form discrete lumps or masses which become too heavy to be entrained or captured in the flow of combustion gases through the furnace, and these larger particles also fall to the bottom. In the wet bottom furnace, they simply fall into the slag pool and lose their individual characteristics. However, in the dry bottom furnace, they generally retain their spherical or agglomerated-spherical identity. Thus, while dry bottom ash may have in it some angular parts which have a porous surface, a substantial portion of the dry bottom ash has the appearance of a fine sand and when examined under magnification, the spherical nature of these particles may be observed and it can be noted that they appear essentially to be internally porous rather than externally porous, and the predominant material is light in color with a sandpaper-like surface texture.
The utilization of both wet bottom slags and dry bottom ashes, particularly in the art of highway construction, as an aggregate in cement stabilized road bases, is described in Seals et al, "Bottom Ash: An Engineering Material", Journal of Soil Mechanics and Foundations Divisions, Proceedings of the Society of Civil Engineering, April, 1972, pp. 311-325; Moulton et al, "Utilization of Ash from Coal Burning Power Plants in Highway Construction", reprint from Highway Research Record 430 by the National Ash Association Inc. (undated); Moulton, "Bottom Ash and Boiler Slag, Ash Utilization Proceedings", Third International Ash Utilization Symposium, U.S. Bureau of Mines Information Circular 8640, pp. 148-169, 1973; and Blocker et al, "Marketing Power Plant Aggregates as a Road Base Material", U.S. Bureau of Mines Information Circular 8640, pp. 208-233.
In the dry bottom furnace, the ash is collected, as noted above, in an essentially dry state and is dumped or collected in a water-filled ash pit. From there it is carried by water from time to time or continuously into a sluicing pond. Frequently a pair of crusher rollers at the ash pit crush the larger chunks of ash down to a size approximately 11/2 inch maximum. In the sluicing pond, the ash is eventually drained of its water content as the pond fills, and the solids may be removed and hauled in dump trucks to a land fill.
During the burning of the coal, some of the ash tends to accumulate on the surface of the boiler tubes. This material is known to be rich in sulfur trioxide. From time to time, the tube ash is removed by water or steam jets or by acid bath, where this material falls to the collection apparatus at the bottom of the furnace and is then sluiced out into the pond with the bottom ashes.